Any such plan will be a 'roaming' plan so not ideal.
In your situation you would normally buy a local SIM in NZ (pre or post paid) and use that for your time over there. You would have another NZ number.
You should possibly look to cancel your Optus plan if that saves you money (do you know what exit fees it has etc.?) If you wanted to keep the mobile number then you would get a cheap long expiry plan from somewhere like Amyasim and port your number across. It costs $10 to park your number for a year.